Improve connection handling logic in ConnService

Connection for HFP HF role in Bluetooth is currently trying to
handle all cases of connected vs disconnect HFP. This can be offloaded
to ConnServ where the connection lifecycle should be maintained (and not
in the connection itself).

1. Fixes NPEs generated by the BtCarHfpFuzzTest.
2. Reduces the number of calls to AT+CLCC

Change-Id: Ibccadb7721cbbd2260cbfc5685d66027277e8a59
(cherry picked from commit aa09a41a685e957786b69d4101f5107b9d6d1d9b)